The present invention relates to an inkjet printhead comprising at least one ink channel connected to a nozzle and defined between two opposing side walls; a membrane interconnecting the side walls; and actuating means arranged adjacent to the membrane for deflecting the membrane.
A conventional printhead of the type, which is disclosed for example in EP-A-0 819 524, comprises a plurality of ink channels which are arranged side-by-side so that the associated nozzles form a linear nozzle array. The ink channels and the nozzles are formed by grooves cut into the surface of a substrate which may, for example, be made of silicon. The membranes for the various ink channels are formed by a continuous thin sheet which is overlaid on the substrate so that it covers the open top sides of the grooves. This sheet has to be firmly bonded to the regions of the substrate defining the side walls of the ink channels. The actuators are formed by piezo-electric fingers which are supported by a common backing plate and have their respective free ends bonded to the top surfaces of the membranes, so that, when a voltage is applied to one of the piezo-electric fingers, the corresponding membrane is flexed into the volume of the associated ink channel which is filled liquid ink, so that an ink droplet is expelled from the nozzle.
U.S. Pat. No. 4,657,631 discloses another type of printhead in which the ink channels are formed by groove-like structures in a metal layer which is formed on the flat surface of the substrate. Thus, the bottom wall of each ink channel is formed by a portion of the substrate, and rigid side walls and a rigid top wall are formed by the metal layer. The actuators are disposed inside of each ink channel and are disposed on the surface of the substrate, so that they are directly exposed to the ink liquid without a membrane intervening between the actuator and the ink. This type of printhead can be manufactured by forming a photo-sensitive layer on the surface of the substrate and by exposing and developing this layer, thereby forming a pattern of ridges which have a shape complementary to that of the ink channels. A metal layer is then formed on the surface of the substrate by sputtering and subsequent electronic plating, until the ridges are buried in the metal layer, the photo-sensitive material is then removed so that the ink channels are formed in the metal layer.
It is an object of the present invention to provide an inkjet printhead which can be manufactured more easily and with a higher production yield. According to the present invention, the side walls and the membrane are formed by a one-piece member. This construction has the advantage that, on the one hand, the membrane can be made very thin so that it has a higher flexibility, and, on the other hand, the member forming the membrane and the side walls of the ink channels can, as a whole, have a comparatively high structural strength. This greatly facilitates the manufacturing process and makes it possible to achieve a high production yield.
Advantageously, the one-piece member which forms the membranes and the side walls of the plurality of ink channels is a metal foil which is obtained in a growth process such as electroforming. This has the advantage that the membrane can be made very thin and the thickness thereof can be controlled with high accuracy. Further, since the foil can be formed directly on the surface on the substrate, no assembly process or separate bonding step is required for forming a complete channel plate, including the substrate, and the metal foil which forms the membrane and the side walls of the ink channels.
By forming the metal foil on the surface of a photoresist which has appropriately been shaped by means of photolithographic techniques, it is possible to obtain a three-dimensional structure of the foil which forms not only the membrane and the side walls of the ink channels but also the nozzles, with appropriate sizes and shapes. The cross-section of the foil in a plane normal to the axis of the ink channels has a meandering-like shape, with a space formed between the portions of the foil which define the side walls of two adjacent ink channels. This construction greatly reduces the amount of mechanical or acoustic coupling between the adjacent ink channels, so that cross-talk among the various channels of the printhead is reduced significantly. In addition, this cross-sectional shape of the foil has the effect that the thermal expansion of the channel plate is controlled only by the material of the substrate, e.g. silicon, which is particularly useful when the printhead is used for hot-melt ink and, accordingly, operates at high temperatures. Since the nozzles are formed directly by the three-dimensionally structured foil, no mechanical finishing of the nozzle front of the printhead is necessary.
By providing a thin metal layer on the surface of the substrate on which the foil is applied in a later manufacturing step, it is possible to obtain a design in which the ink channels and, more important, also the nozzles are completely surrounded by only one type of material, i.e. metal, so that the directionality of the droplet-formation process will not be influenced by differences in the adhesiveness of the walls defining the nozzles.